Mercury is a well-recognized health hazard and an environmental contaminant. Mercury modulates immune responses ranging from immune suppression to autoimmunity but the mechanisms responsible for these effects are still unclear. Male BALB/c mice were exposed continuously to 0, 0.3, 1.5, 7.5, or 37.5 ppm mercury in drinking water for 14 days. Body weight was reduced at the highest dose of mercury whereas the relative kidney and spleen weights were significantly increased. The dose range of mercury used did not cause hepatotoxicity as indicated by circulating alanine aminotransferase and aspartate aminotransferase levels. Circulating blood leukocytes were elevated in mice treated with the highest dose of mercury. Mercury ranging from 1.5 to 37.5 ppm dose-dependently decreased CD3(+) T lymphocytes in spleen; both CD4(+) and CD8(+) single-positive lymphocyte populations were decreased. Exposure to 7.5 and 37.5 ppm mercury decreased the CD8(+) T lymphocyte population in the thymus, whereas double-positive CD4(+)/CD8(+) and CD4(+) thymocytes were not altered. Mercury altered the expression of inflammatory cytokines (tumor necrosis factor alpha, interferon gamma, and interleukin-12), c-myc, and major histocompatibility complex II, in various organs. Results indicated that a decrease in T lymphocyte populations in immune organs and altered cytokine gene expression may contribute to the immunotoxic effects of inorganic mercury.